The cinema sound industry is currently undergoing a significant change, from widespread use of multi-channel loudspeaker systems having a small number of channels (e.g., 5.1 or 7.1 channel systems having five or seven full-range channels) to use of new systems that provide many more channels (typically, N full-range channels, where 12≦N≦64). Such new systems, in which loudspeakers are typically located over the whole hemisphere above listeners, allow precise location and motion of sounds within the hemisphere, and can recreate more realistic “3D” ambiences and reverbs. Herein, we will sometimes use the expression “many-channel system” (in contrast with “multi-channel” system) to refer to a system of the new type, in which the number of full-range channels is much greater than 7.
It is expected that, in typical use, many-channel systems will pan sound sources based on amplitude-panning which, for a given sound source, strongly depends on the coherence in the signals arriving from the few loudspeakers (a subset of the large set of installed loudspeakers) which participate in the reproduction. Even in systems as simple as stereo, the perceived location of a sound intended to be panned between speakers can be rendered vaguely, or even outside the area between the speakers, if the responses (amplitude and phase) of the two speakers are incorrectly matched.
It is therefore essential for the current worldwide deployment of the new many-channel speaker systems to have technology available for ensuring that all channels in a given playback venue are properly matched. Most existing equalization processes focus on correcting the amplitude response of the different channels, which ensures a correct match of timbre perception across channels. However, to ensure proper sound imaging across the entire system, the matching of the phase response of each channel needs to be addressed.
One of the most common problems encountered in many-channel installations is that the polarity of a number of channels is inverted. This is normally due to either incorrect wiring during the set up stage, or to incorrect wiring inside one of the components of the audio chain. The latter is more difficult to detect and fix by the installer, as all visible wiring is actually correct. In both cases, however, the sound imaging will be seriously compromised when channels having incorrect speaker polarity participate in sound panning.
Furthermore, in a multi-way active or passive loudspeaker system (having multiple drivers), polarity inversion can affect only one of the drivers. When wrong polarity takes place in the bass driver, the sound imaging can be as severely compromised as when the whole loudspeaker polarity system is inverted, as well-known in the psychoacoustics literature. It is therefore important to ensure correct polarity matching not only across channels, but also across different drivers in a single channel.
It is important to implement loudspeaker polarity detection to be automatic and to avoid taking extra time. The inventors have recognized that in order to implement quick and automatic loudspeaker polarity detection, the use of tone bursts or asymmetric signals (as in the paper D. B. Keele, Jr., “Measurement of Polarity Band-Limited Systems,” presented at the 91st Audio Engineering Society Convention in New York, Oct. 4-8, 1991) should be avoided.
With the expected increase of the number of channels to be installed in typical playback venues, the possibilities of wrong-polarity problems increase accordingly. Unfortunately, the time required to set up a many-channel speaker system may be long. As a result, it is expected that many-channel system installers will often have less time to check and correct wrong-polarity issues. Therefore, it would be desirable to provide methods that, on one hand, perform such checks automatically, and on the other hand, do not have a significant impact on the time needed for setting up. The latter restriction favors methods that do not require the emission and capturing of additional signals specifically tailored for polarity analysis, and instead are capable of re-using the measurements normally performed during conventional initial calibration or alignment (sometimes referred to as equalization or theater equalization) of a newly installed speaker array.
Finally, it is desirable that automatic methods for determining loudspeaker polarity be robust to choices of the type, and position(s) in a playback venue, of the measuring microphone(s), as well as robust to natural differences in the details of the phase response due to the presence of different loudspeaker models in the venue and differences in the positions of the loudspeakers in the venue. Unfortunately, delays, reverberation, and noise have made conventional polarity checking methods inaccurate and/or otherwise problematic.
A conventional method for automatic determination of loudspeaker phase is described in US Patent Application Publication No. 2006/0050891, published on Mar. 9, 2006. This method includes steps of driving a speaker with an impulse, capturing the resulting emitted sound using a microphone, determining an impulse response (from the speaker to the microphone) from the captured audio, and determining polarity of the speaker by determining the sign of the first peak of the impulse response (the first peak having an amplitude whose absolute value exceeds a predetermined threshold). If the sign of the first peak's amplitude is positive, the method determines that the speaker has positive polarity. However, this method is subject to the limitation that it does not determine quality of the measured impulse response, and thus can undesirably determine a speaker polarity from a wrongly measured response (e.g., a response indicative of noise only).